1. Field of the Invention
The present invention relates to WLAN (Wireless Local Area Network) communication devices and corresponding methods and integrated circuit chips, and in particular to the backoff generation in such WLAN communication devices.
2. Description of the Related Art
A wireless local area network is a flexible data communication system implemented as an extension to or as an alternative for a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Today, most WLAN systems use spread spectrum technology, a wide band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to 802.11b that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. Further extensions exist.
One example of these is the 802.11e extension, also referred to as WME (Wireless Media Extensions), that was designed to address QoS (Quality of Service) issues of the precedent 802.11 versions. For this purpose, the 802.11e specification provides MAC (Medium Access Control) enhancements to meet the QoS requirements of multimedia applications like voice over IP or audio/video streaming.
The previous 802.11 MAC layer had no means of differentiating traffic streams or sources. As a result, no consideration could be made for the service requirements of the traffic on the channel. The 802.11e specification introduces two new MAC modes, EDCF (Enhanced Distributed Coordination Function) and HCF (Hybrid Coordination Function), which support up to eight priority traffic classes (TCs).
Referring now to the figures and in particular to FIG. 1, a WLAN communication device, i.e., a transmitter or transceiver is shown in which a number n of traffic classes 105, 130, 155 is implemented. For each traffic class 105, 130, 155, the WLAN communication device includes a FIFO (First In First Out) storage 110, 135, 160 in which packets to be transmitted are queued. Each traffic class having packets to transmit starts a backoff operation after detecting the channel being idle for an arbitration interframe space (AIFS) which can be chosen individually for each traffic class and provides a deterministic priority mechanism between the traffic classes.
The following backoff operation is quantized into so-called time slots. Also, the AIFS interval is usually indicated as an integer number of time slots. A backoff counter 125, 150, 175 assigned to each traffic class 105, 130, 155 is decreased once every time slot. When the backoff counter value of a traffic class reaches zero, the respective traffic class attempts to transmit a packet out of its queue 110, 135, 160. For the next backoff operation, the backoff counter 125, 150, 175 is then set to a BC (Backoff Counter) start value selected randomly by the backoff generator 120, 145, 170 out of a contention window (CW). If, however, the backoff counter value has not reached zero before the channel becomes busy again, the backoff counter value is frozen and the next backoff operation is started with this value.
The minimum initial value of the contention window, denoted by CWmin, can be selected on a per TC basis. As collisions occur, the contention window is multiplied by a persistence factor (PF) that can be chosen individually for each traffic class 105, 130, 155 in the CW adaptors 115, 140, 165, thus providing a probabilistic priority mechanism between the traffic classes 105, 130, 155. Optionally, a maximum possible value CWmax for the contention window can also be selected individually for the traffic classes 105, 130, 155.
Within the WLAN communication device, the traffic classes have independent transmission queues 110, 135, 160. These behave as virtual stations within the above-mentioned parameters AIFS, CWmin, CWmax, and PF, determining their ability to transmit. If the backoff counter of two or more parallel traffic classes 105, 130, 155 in a single WLAN communication device reach zero at the same time, a packet scheduler 180 inside the WLAN communication device treats the event as a virtual collision without recording every transmission. A transmit opportunity is given to the traffic class 105, 130, 155 with the highest priority of the colliding traffic classes, and the others back off as if a collision on the medium occurred.
As can be seen from FIG. 1, each traffic class 105, 130, 155 has assigned its own backoff generator 120, 145, 170. This causes the conventional architecture to be unnecessarily hardware consuming. Traditional WLAN communication devices therefore often suffer from the problem of increased manufacturing costs. Further, this prior art layout is less suitable for device miniaturization, e.g., when aiming at providing WLAN compatible mobile telephones or PDAs (Personal Digital Assistants).